I just wanted to chime in and say that you're not crazy: I've been having similar issues with Chromium, the FP 11.5 beta, and YouTube too. (You linked to the same video twice, BTW, so I'm not sure if I saw the broken one or not. (It played.))

Chromium

25.0.1338.0 (Developer Build 169999)

OS

Windows 8

WebKit

537.20 (@136009)

JavaScript

V8 3.15.6

Flash

11,5,502,131

On the other hand, I can't duplicate the problem today, so maybe it was YouTube's fault.